This invention relates generally to x-ray transmission and radionuclide emission imaging systems. In a primary application the invention relates to a diagnostic imaging system where x-ray transmission data are used to derive material properties of the object imaged, which properties are then used directly to correct radionuclide emission data obtained from the same object using the same imaging system, herein referred to as X-SPECT.
Diagnostic imaging techniques can image both anatomical structure and physiological function of patients in whom the physician wishes to diagnose disease or follow treatment. In clinical settings, most of these diagnostic imaging techniques--including conventional film radiography. MRI, and CT--predominantly image structure rather than function. Unlike these anatomical imaging methodologies, in vivo measurement of tissue metabolism, perfusion, and biochemical processes is best performed with emission radionuclide-tracer techniques. In these techniques, a radionuclide or a compound labeled with a radionuclide is injected into a subject. The radiolabelled material concentrates in an organ or lesion of interest, and can show a concentration defect. At a prescribed time following injection, the pattern of concentration of the radiolabelled material is imaged by a rectilinear scanner, scintillation camera, single-photon emission computed tomography (SPECT) system, or positron emission tomography (PET) system. Applications of radionuclide imaging include quantitation of tissue metabolism and blood flow, evaluation of coronary artery disease, tumor and organ localization and volume determination, quantitation of receptor binding, measurement of brain perfusion, and liver imaging.
The radionuclide imaging procedure requires a means to define the path along which the emitted gamma-ray travels before striking the detector of the imaging system. The path can be a vector path, a line, narrow fan, or a narrow cone as defined by the detector or collimator. In rectilinear scanners, scintillation cameras, and SPECT systems, a collimator (typically made of lead or other high-atomic number material) is interposed between the object and the detector to define the gamma-ray path. In PET, the unique characteristics of positron annihilation radiation are coupled with electronic circuitry to define the vector path. In all cases, the only information obtained when a gamma-ray strikes the detector is the fact that the photon originated somewhere within the object along the vector path projected back from the detector. For projection imaging systems, a two-dimensional image is formed with the intensity of each picture element, or pixel, proportional to the number of photons striking the detector at that position. In SPECT or PET, the vector paths are determined for multiple projection positions, or views, of the object, and cross-sectional or tomographic images are reconstructed of the object using standard algorithms. Again, the intensity assigned to each vector path is proportional to the number of photons striking the detector originating along the path, and the intensity of each pixel in the reconstructed image is related to these vector path intensities obtained at multiple views.
In radionuclide imaging, it is desirable to obtain absolute values for radionuclide concentrations (or radionuclide uptake) at each point in the image. Attenuation of the emitted photons within the object, before they reach the detector, is a function of the energy of the photons and the exact composition of the material through which the photons pass to reach the detector. Photons emitted deeper within the object have a higher probability of attenuation than those emitted near the surface. In addition, the composition of the material (in terms of effective atomic number Z and electron density) affects the attenuation, with more attenuation if the path passes through high-Z or high-density regions. Thus, in order to calculate absolute uptake or concentration of a radionuclide in a region of an object, it is required that the path length of each type of material or tissue (or effective-Z and electron density path lengths) be known for each vector. Attenuation corrections for emitted photons are made from this knowledge, allowing accurate concentration values to be obtained.
The full clinical potential of radionuclide imaging has been seriously hindered by some important limitations. The spatial resolution and photon statistical limitations of radionuclide imaging frustrate accurate anatomical localization and hinder quantitation of the radionuclide distribution. Photon attenuation has been identified by the American Heart Association and leading nuclear cardiologists as a major deficiency in diagnosis of heart disease with SPECT, and is a major source of error in the measurement of tumor metabolism using radionuclide techniques. Quantitation is further complicated by the need for scatter compensation for imaging with both single-photon and positron-emitting radionuclides.
A number of researchers have shown that many of these limitations can be overcome through use of emission-transmission imaging techniques which combine anatomical (structural) information from transmission images with physiological (functional) information from radionuclide emission images. By correlating the emission and transmission images, the observer can more easily identify and delineate the location of radionuclide uptake. In addition, the quantitative accuracy of measurement of radionuclide uptake can be improved through use of iterative reconstruction methods which can account for these errors and improve the radionuclide images.
Currently existing medical imaging instrumentation has been designed for either emission or transmission imaging, but not both, and attempts to perform both compromise one or both of the data sets. In addition, as currently implemented, iterative reconstruction algorithms are too slow to converge and therefore impede the flow of information in a hospital setting. Virtually all clinical tomographic systems use analytic rather than iterative reconstruction algorithms which, unlike iterative reconstruction techniques, have the major advantage that the image reconstruction process can occur concurrently with the acquisition of the image data. The efficiency of analytic approaches is compromised by their inability to account for the quantitative errors of photon attenuation, scatter radiation, and spatial resolution losses mentioned above.
The prior art in this field includes several different approaches to localize and quantify the uptake of radionuclides in the human body. One approach uses stereotactic techniques or computer processing methods to correlate functional information from SPECT or PET images with morphologic information from magnetic resonance imaging (MRI) or CT. This technique has the advantage that it can be applied retrospectively without acquiring new image data from the patient. However, these approaches are computationally intensive, require that the patient be scanned separately on two systems, and have only been successful in the head where the skull limits motion of internal anatomical structures.
A second set of prior art describes instrumentation used to detect emission and transmission data using instruments with single or multiple detectors. Several investigators have acquired both the emission and transmission images, with a radionuclide point, line, or sheet used as the transmission source which is placed on the opposite side of the body from the scintillation camera. This approach has been applied more recently using SPECT. Studies have shown that this technique is capable of producing adequate attenuation maps for attenuation correction to improve quantitation of radionuclide uptake, and that some modest anatomical localization of the radionuclide distribution is also possible.
An alternative approach uses specially-designed instruments for emission-transmission imaging. For example, Kaplan (International Patent Application No. PCT/US90/03722) describes an emission-transmission system in which the emission and transmission data are acquired with the same detector (single or multiple heads). An alternative emission-transmission imaging system (disclosed in SU-1405-819-A) uses x-ray transmission data and two detectors for determining the direction of the photons to improve detection efficiency. However, an exact method of correcting emission data based on transmission data is not described by either Kaplan or in SU-1405-819-A.
Other prior art notes that the map of attenuation coefficients required for the attenuation correction procedure can be obtained from a separate x-ray transmission CT scan of the patient, although a specific method of generating an attenuation map at the photon energy of the radionuclide source is not known. Specific techniques to determine the attenuation map of the patient from single-energy transmission measurement using radionuclide or x-ray sources have been described which are limited to sources emitting monoenergetic (line) spectra rather than broad spectra such as those typically obtained from an x-ray source.
Specific algorithms for correcting beam-hardening artifacts use single-energy x-ray data and dual-energy x-ray data. As used herein, the term "single-energy x-ray" describes methods in which an image is generated by integrating the x-ray signal over a single range of photon energies. As used herein, the term "dual-energy x-ray" describes methods in which two images are generated by integrating the signal over two different photon energy ranges. Thus, either "single-energy x-ray" or "dual-energy x-ray" includes methods in which the x-ray source emits an x-ray beam having either a narrow of broad spectrum of energies. Algorithms for correcting beam-hardening artifacts by using basis-material measurements derived from single-energy or dual-energy x-ray data have been presented but without describing how these measurements can be applied to correction of radionuclide data. Especially for single-energy measurements, the correction techniques associated therewith are principally directed at the removal of beam-hardening streaks and nonuniformities which disturb the qualitative evaluation of images produced with CT.
A key element has been the combination of the emission and transmission data in a reconstruction algorithm which corrects the radionuclide distribution for photon attenuation. Several authors have described analytic algorithms such as filtered backprojection in which the radionuclide data is modified using an attenuation map to correct for attenuation errors. Among their advantages, these analytic algorithms are fast and require only a single step to reconstruct the radionuclide distribution. However, they are inexact and utilize a uniform attenuation map in which the value of the attenuation coefficient is assumed to be constant across the patient. Other reconstruction algorithms are iterative and use an exact attenuation map and the radionuclide projection data to estimate the radionuclide distribution across the patient. Maximum likelihood estimation is one statistical method that can be used for image reconstruction. A maximum likelihood estimator appropriate for radionuclide tomography based on an iterative expectation maximization algorithm (ML-EM) has been described. The ML-EM algorithm is easy to implement, accounts for the Poisson nature of the photon counting process inherent with radionuclide imaging, and it produces better images than filtered backprojection. In addition, ML-EM algorithms can incorporate physical phenomena associated with radionuclide tomography, such as photon attenuation and scatter, detection efficiency, and geometric aspects of the imaging process. Iterative weighted least squares/conjugate gradient (WLS/CG) methods have also been proposed and used for radionuclide tomography. Overall, WLS/CG reconstruction algorithms converge faster than ML-EM procedures, while still incorporating the statistical nature of radionuclide imaging, and permit compensation for photon attenuation and scatter, detection efficiency and geometric response. Iterative algorithms have been successfully used for both SPECT and PET imaging.
The major disadvantage of iterative algorithms is their computational burden. Iterative algorithms are iterative procedures and are started with an initial image estimate that either corresponds to a constant radionuclide density throughout the image plane to be reconstructed or corresponds to constant density throughout the highly sampled "reconstruction circle" and zero outside this region. This estimate is unlikely to be representative of the actual distribution of radionuclide in a patient, and a large fraction of the total iterations required to generate useful images may be necessary to reveal the real qualitative structure of the radionuclide distribution. Thus, these algorithms often require 30 to 50 iterations to yield visually acceptable images, and possibly several hundred iterations to generate quantitatively accurate reconstructions.
It also is possible to use filtered backprojection to produce initial image estimates for iterative reconstruction algorithms. Filtered backprojection algorithms can operate concurrently with the emission data acquisition, and they are the method currently used for most clinical radionuclide imaging systems due to their efficiency and ability to produce useful images. Unfortunately it is generally not possible to modify filtered backprojection algorithms to accurately account for details of the collimator geometry, or for the effects of scatter, especially in regions where there are large inhomogeneities in these properties, or details of the collimator geometry. Therefore, this approach can speed up iterative techniques slightly, although the improvement in convergence speed has not been dramatic. Thus, many investigators have pursued various methods of speeding the convergence of ML-EM algorithms or reducing the time required per iteration. Methods include exploiting the symmetry of the imaging system, multigrid approaches, high frequency enhanced filtered iterative reconstruction, expectation maximization search (EMS) algorithms, rescaled gradient procedures, vector-extrapolated maximum likelihood algorithms, and hybrid maximum likelihood/weighted least squares (ML/WLS) algorithms. However, all iterative reconstruction methods require significantly more computer time than filtered backprojection algorithms to generate useful images. The iterative ML-EM and WLS/CG algorithms mentioned above assume complete sets of radionuclide projection data exists prior to commencement of the reconstruction procedure. The requirement to acquire complete sets of projection data is especially important in radionuclide system because clinical emission imaging systems typically require several minutes to acquire projection data, making iterative reconstruction techniques impractical.